In French, "c'est" (sing.) and "ce sont" (plural) are used every time pronoun it, she, he or they is subject of verb "être" and followed by a nominal group, ie: modifier (1) + noun (+ adjective)
o it is + noun => c'est + modifier + noun (+ adjective)
o she is + noun => c'est + modifier + noun (+ adjective)
o he is + noun => c'est c'est + modifier + noun (+ adjective)
o they are + noun => ce sont + modifier + noun (+ adjective)
(1)NOTE: French nouns are always used with "modifiers": articles, definite or indefinite (le/la/les, un/une/des) or possessive adjectives (mon/ma/mes, etc) or demonstrative adjectives (ce/cette/ces) or numeral (deux, trois...).
(2)NOTE: the above rule has no exception with indefinite article un/une/des, but a few exceptions with other modifiers:
- he is THE chief = IL est LE chef (single statute) + c'est LE chef
- she is HIS second wife = ELLE est SA seconde épouse (single statute) + c'est SA seconde épouse
they are French = ils sont français or elles sont françaises (adjectives)
they are French men = ce sont des Français (noun)
they are French women = ce sont des Françaises (noun)
they are French = ils sont français OR elles sont françaises (nationality as an adjective)
they are Frenchwomen = ce sont des Françaises (women / nationality as a noun)
they are French ones = ce sont des Françaises (things or animals / nationality as a noun) - I'm pretty sure you would only say "they are French" in this case, but the back translation would be wrong (ils/elles sont français/es - adjective).
True but then you do not typically point that out in English. "they are french" is normal English sentence, regardless the gender. Sometimes you may want to highlight the fact that you are talking about women and use some word (like 'women') in the sentence. But many times you do not have to as it is not needed or the audience already knows the objects are females. You would never use masculine form though in French, cause it would be simply silly even if the audience already knows you are referring to a group of females. I think "They are French" should be acceptable.